1. Field of the Invention
The present invention relates to an optical information recording medium. More particularly, the present invention relates to an optical information recording medium which can record, reproduce or rewrite information at a high recording density and a high speed using an optical means such as a laser beam.
2. Description of the Related Art
A technique for recording and reproducing information at a high recording density on an optical disc using a laser beam and the like is known and applied to a document file, a still picture file or an external memory of a computer. A rewritable information recording system has been studied and developed. One example of such system is a phase-change optical disc, which records and erases signals using a reversible phase change in a recording layer between an amorphous state and a crystalline state of a compound or between one crystalline state and another crystalline state of a compound by the irradiation of a laser beam. As a recording material in which the reversible phase change occurs, an alloy of Te, Se, In or Sb is mainly used. When such alloy is heated and molten and then quenched, it becomes amorphous, and when the amorphous alloy is heated and kept at a temperature of its crystallization temperature or higher or molten and quenched, it is converted to the crystalline state. Since the reflectance is different between the amorphous state and the crystalline state, the information is reproduced by optically detecting such reflectance difference.
In general, the phase-change optical disc has a three-layer structure comprising a recording layer which is sandwiched by a pair of protective layers containing a dielectric material and laminated on a substrate or a four-layer structure comprising a reflective layer formed on the above three-layer structure.
In the three-layer structure, the recording layer usually has a thickness of 70 nm to 120 nm to increase the change of the reflectance between the crystalline state and the amorphous state. In the four-layer structure, the recording layer has a thickness of 20 nm to 50 nm to utilize an enhancing effect by the reflective layer and the dielectric material.
As a special structure of the recording medium, Japanese Patent Kokai Publication No. 187950/1985 discloses a structure comprising alternately laminated metal films each having a thickness of 2 to 20 nm and silicon oxide films each having a thickness of 1 to 40 nm to increase sensitivity of the recording medium and chemical stability. In this recording medium, various metals including Sb.sub.2 Te.sub.3, Co, Pt and Te are exemplified as the material to be used as the recording layer. However, this Japanese Patent Kokai Publication does not suggest or limit a composition for rewritable recording.
One of characteristics required for the phase-change optical disc is a high recording-erasing sensitivity. This means that the signals can be recorded and erased with the irradiation of the laser beam having a low power and that a cost of the recording apparatus can be reduced. The phase change from the crystalline state to the amorphous state (in general, a signal recording step) requires the largest laser beam power in case of, for example, a medium in which the phase change is carried out between the amorphous state and the crystalline state, since the amorphous is generated by heating and melting the crystalline and then quenching it. However, in general, a material which is easily made amorphous is difficult to crystallize, so that the erasure of the signal is difficult. To overcome this drawback, a medium having a high crystallization sensitivity is used to achieve a necessary erasing sensitivity, while a laser beam with a large power is applied to record the signals. That is, in such case, a large power laser beam is required.
Another one of the characteristics required for the phase-change optical disc is thermal stability of the recorded signal, namely stability of the amorphous state. Even a material which is expected to change its phase between the amorphous state and the crystalline state may not be made amorphous by the irradiation of the laser beam on a thick film of the crystal or, if it is made amorphous, it may be readily crystallized. Such material cannot be used in the phase-change optical disc. This may be largely because the crystallization temperature of such material is around room temperature. This is the reason why the study has been concentrated on an alloy of Te having a composition near an eutectic composition, an alloy of Se, a composition of a Ge-containing compound and a composition in which the amorphous phase is stable even in case of a single recording layer. However, a compound having a stable amorphous phase is not necessarily excellent in other characteristics, and the number of materials which can be practically used as a recording medium is very small.